1. Field of the Invention
The present invention relates to a film thickness-measuring apparatus suitably used for measuring the film thickness of an object to be measured, which is moved at high speed, e.g., the film thickness of an oil coated on a steel plate in a rolling line or a plating line, or the film thicknesses of other thin films, in an on-line manner.
2. Description of the Related Art
Ellipsometry is generally used as a means for measuring the film thickness of a thin film having a film thickness of several 1,000 .ANG.. According to this method, a change in polarization state when light is reflected by a thin film sample surface, i.e., ratio .rho. of Fresnel complex amplitude reflection coefficient rp of one electrical vector component (p component) parallel to an incidence plane to Fresnel complex amplitude reflection coefficient rs of the other component (s component) perpendicular to the incidence plane is obtained by equation (1) below, and film thickness d is obtained in accordance with a predetermined function of already established reflection coefficient ratio .rho. and film thickness d. EQU .rho.=rp/rs=tan .psi.e.sup.j.DELTA. ( 1)
Since reflection coefficient ratio .rho. is a complex number, two ellipsometric parameters, i.e., amplitude ratio tan .psi. and phase .DELTA. must be obtained. One of conventional means for obtaining these two ellipsometric parameters .psi. and .DELTA. at high speed is disclosed in U.S. patent application Ser. No. 62,242.
According to this means, linearly polarized light formed by a polarizer and having a predetermined azimuth is caused to be incident on the incidence surface of an object to be measured at a predetermined angle. Then, the light reflected by the object is converted into a plurality of light beams using a beam splitter section. The plurality of light beams are caused to pass through a plurality of analyzers having different transmission polarization azimuths, and are respectively focused by a plurality of focusing lenses at predetermined focal lengths. The plurality of light beams focused by these focusing lenses are detected by a plurality of photodetectors through pinholes located at the respective focal points, thereby outputting electrical signals corresponding to the respective light amounts/intensities. Then, the respective electrical signals from these photodetectors are subjected to a predetermined arithmetic operation so as to obtain two ellipsometric parameters, i.e., amplitude ratio tan .psi. and phase .DELTA.. With this operation, two ellipsometric parameters .psi. and .DELTA. can be reliably obtained using a fixed optical system. The film thickness or refractive index of the object to be measured can be calculated on the basis of these parameters.
As described above, the method disclosed in U.S. patent application Ser. No. 62,242 exhibits excellent characteristics compared with other conventional methods. However, when the film thickness of a transparent film formed on a transparent glass substrate is measured, the following problems are posed. In the invention disclosed in Ser. No. 62,242, linearly polarized light is used as incident light, reflected light is branched into three beams by a beam splitter constituted by three or more parallel flat optical flats, analyzers having predetermined transmission axis angles with respect to the branch beams are arranged, and the light amounts of the three branch beams after passing through the analyzers are measured. In this case, as is apparent from the description of the embodiment of this application, ellipsometric parameter .DELTA. is obtained from the intensities of the three light beams I1, I2, and I3 as phase cosine cos .DELTA.. In this case, if a polarizer azimuth is set to be 45.degree., analyzer azimuth .alpha.1=0.degree.,.alpha.2=45.degree., and .alpha.3=-45.degree., then ##EQU1##
When the optical flats are transparent, .phi.0=0.degree.. Thus, cos (.DELTA.-.phi.0)=cos .DELTA. according to equation (2). Furthermore, ellipsometric parameter .DELTA. may become close to 180.degree. when, e.g., the film thickness of a transparent film formed on a transparent glass substrate or the like is measured. In such a case, since the rate of change in cos .DELTA. is close to 0 when .DELTA. is close to 0.degree. or 180.degree., even if a small error is included in the right side of equation (2), an error in .DELTA. becomes large, and hence accurate measurement cannot be performed.